The invention relates to distance detecting apparatuses and, in particular, to ultrasonic depth sounders, also known as sonars.
Various types of distance measuring apparatuses work on the principle of sending out a signal, and then measuring the time delay which occurs before a reflected signal is received back. The time interval is proportional to the distance between the point where the signal is transmitted and the object from which the signal is reflected. Ultrasonic depth sounders, typically used on sports or commercial fishing boats, are examples of such distance measuring devices. These basically include a transmitter for generating an electrical signal, a transducer for converting the electrical signal into an ultrasonic signal, for transmitting the ultrasonic signal into the water, and also for receiving the reflected ultrasonic signal and converting it back into an electrical signal. A receiver receives the return electrical signal from the transducer. There is a display device for converting the time delay into a readable form which indicates the depth from the transducer, typically in the bottom of the boat, to the fish, bottom or other objects below the boat.
Basically these devices measure the time it takes for a signal to return. Sound travels at a known rate in water with small variations, depending upon the salinity of the water, and so the depth is proportional to the time between the transmitted signal and the received echo. Clearly, therefore, some control is necessary of the transmit signal in order to permit calculation of the time and therefore the distance.
One of the earliest types of devices is still on sale, and is known as a "flasher". The flasher employs a rotating ring with contacts which trigger the transmit cycle once per revolution. A neon tube is mounted on the ring, and is fired by the amplified received echo. The distance the ring rotates from the transmit position before the neon is illuminated is proportional to the time between the transmit signal and the return of the echo and therefore is proportional to the depth. However, such devices can only indicate instantaneous depth.
Chart recording depth recorders are used to show a record or trend in the depth. A pen moves across a roll of paper and presses against the paper when an echo is received. The paper is then advanced and the cycle repeated.
A more recent development has been the use of a video display, either a cathode ray tube (CRT) or a liquid crystal display (LCD). These provide a record similar to a track recorder, except that the image is lost once the picture reaches the end of the screen.
Microprocessers have been used on depth sounders to make them easier to use and, with some signal processing, provide displays which are clear and easier to interpret.
As mentioned, the ultrasonic transducer converts the high frequency electrical signal into an ultrasonic signal which is transmitted into the water during the transmitting phase of operation of the depth sounder. The transmitting phase is followed by a receiving phase when the reflected ultrasonic signals reach the transducer again. The transducer then converts the ultrasonic echo back into electrical signals which are filtered, amplified and converted to a visual display as described above. Transducers are usually fitted into a permanent position in the bottom of a boat, and therefore are not easily removeable. A main unit housing the transmitter, receiver and display device is usually located at a convenient place on the boat and is connected to the transducer by a cable. If the main unit fails, or if the boat owner wishes to update the unit, he is faced with the problem of compatability between the main unit and the transducer, because not all transducers are the same.
Firstly, transducers differ as to their resonant frequency, that is the optimum frequency at which they convert electrical signals into ultrasonic signals. There is no optimum frequency for all conditions and thus, any particular frequency chosen is a compromise in design. Most depth sounders have a frequency of 25 KHz-200 KHz. Frequencies of 50 KHz-200 KHz are most common. The lower the frequency, the greater the possible depth from which an echo can be successfully recovered. On the other hand, the higher the frequency, the greater the potential resolution. Therefore, two basic systems are commonly employed, one around 50 KHz for maximum depth and one about 200 KHz for maximum detail. Depth sounders have been built which are convertible from one frequency to another by switching circuit cards having different components for each frequency. The main unit then must be connected to a transducer suited to that particular frequency. Some depth sounders may receive both frequencies simultaneously.
Transducers also vary in the beam angle, that is the manner in which the ultrasonic signal is propagated below the boat. point source transducers would provide acoustic energy radiating equally in all directions in a spherical manner. Cylindrical transducers produce a downwardly expanding cone with a point at the transducer. The strongest and clearest echos are received from objects within the cone, that is, within the beam area. Small transducers result in wide beam angles which cover the widest possible area, while larger transducers can produce narrower angles which cover less area, but produce stronger and sharper echos.
Transducers employ crystals which are generally cut in a way such that the optimum transmit frequency is not exactly the same as the optimum angle for receiving ultrasonic signals. Consequently, there is a frequency between the optimum transmit frequency and optimum receive frequency which produces the best overall results, and this frequency varies from sample to sample of the same type of transducer. Therefore manufacturers may provide the ability to vary the transmit and receive characteristics over a small range to allow tuning in the field to optimize the response for a particular transducer.
Transducers also vary as to the maximum power they can receive from the transmitter, and consequently convert into ultrasonic signals. Increasing power is required to produce a discernable signal over noise at increasing depths. Typically, for example, power of 100 watts RMS might suffice for 500 ft., while 600 watts RMS might be required at 1,000 ft. If too much power is fed to a particular transducer, then cavitation occurs, which dramatically reduces performance or even actual physical damage may occur in the form of fractured crystals. Consequently, it is desirable to provide power which is just below the maximum that a particular transducer can accommodate.
Since transducers are more permanent installations than the main unit or display unit on the boat, and in view of their varying characteristics discussed above, it is an object of this invention to provide transmitter, receiver and display components which are compatible with a wide range of transducers, so that a single unit or a combination of units incorporating these components can be used irrespective of the particular transducer. Economies result from the fact that only a single apparatus needs to be manufactured and stocked by retailers and wholesalers rather than a range of different components for different transducers.
It is also an object of the invention to provide transmitters and receivers capable of changing frequencies over a wide range without the necessity of changing components. The apparatus thus may be adapted for different conditions of the transducer or the environment by simple adjustments, or automatically under microprocesser control, without changing circuit boards and the like.
It is a further object of the invention to provide a transmitter portion for a depth sounding apparatus which has power output controllable to match a particular transducer. This adjustment should be automatic so that the efficiency of the depth sounding apparatus is maximized, without jeopardizing the transducer itself.
It is a further object of the invention to provide a receiver portion with a centre of frequency response which automatically and exactly corresponds with the frequency of the transmitter.
Another object of the invention is to provide a receiver portion with an adjustable band width such that the band width is increased for maximum resolution when noise level increases and is decreased when noise levels increase.